Well I'm not an expert but...
If you use the net you should make sure that your running anti-virus software and its kept uptodate (usually through weekly/fortnightly update downloads) you should also make sure you update windows (if thats what your running) when security updates are released, these are usually called service packs. They fix any problems that have been discovered. Doing this makes your own computer fairly safe.
Similar things need to be done with websites, the computers they are run on need to been kept uptodate with software upgrades and also the forum software needs to be updated if problems are found.
Currently as far as I can tell we're all up todate so I'm not sure how they managed to hack the forum.
If they managed to hack the forum database they potentially have any information you have entered in it. (Passwords are encrypted even I can't view those as administrator) but with this sort of site there aren't really interested in the data, its not like we have credit card details etc. stored.
So far they haven't got into the database anyway, they just effected the files. So this time for example they just replaced the index file with a message saying it had been hacked.
As to why they do it, well I guess they are lots of reasons. To prove they can, the prestige of being able to say to there friends I hacked x sites, for the challenge...
I have backups of the files and do fairly regular backups of the db as do our host so anything they break can be restored with at the most the loss of a few days posts. Its more of an annoyance than anything else.
Hope that answers a few questions
Tam